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function splitBackgroundLayers(input: string): string[] {
const layers: string[] = [];
let depth = 0;
let start = 0;
for (let i = 0; i < input.length; i += 1) {
const ch = input[i] as string;
if (ch === "(") depth += 1;
else if (ch === ")") depth -= 1;
else if (ch === "," && depth === 0) {
layers.push(input.slice(start, i).trim());
start = i + 1;
}
}
layers.push(input.slice(start).trim());
return layers;
}
function extractColorFromStop(stop: string): string {
const s = stop.trim();
const funcs = ["rgba(", "rgb(", "hsla(", "hsl("] as const;
for (const fn of funcs) {
if (s.startsWith(fn)) {
let depth = 0;
for (let i = 0; i < s.length; i += 1) {
const ch = s[i] as string;
if (ch === "(") depth += 1;
else if (ch === ")") {
depth -= 1;
if (depth === 0) {
return s.slice(0, i + 1);
}
}
}
return s;
}
}
const firstToken = s.split(" ")[0] as string;
return firstToken;
}
function drawLinearGradient(
ctx: CanvasRenderingContext2D,
width: number,
height: number,
layer: string
): void {
const inside = layer.slice(layer.indexOf("(") + 1, layer.lastIndexOf(")"));
const parts = splitBackgroundLayers(inside);
const dir = (parts.shift() || "").trim();
let x0 = 0,
y0 = 0,
x1 = width,
y1 = 0;
if (dir.endsWith("deg")) {
const deg = parseFloat(dir);
const rad = ((90 - deg) * Math.PI) / 180;
const cx = width / 2;
const cy = height / 2;
const r = Math.hypot(width, height) / 2;
x0 = cx - Math.cos(rad) * r;
y0 = cy - Math.sin(rad) * r;
x1 = cx + Math.cos(rad) * r;
y1 = cy + Math.sin(rad) * r;
} else if (dir.startsWith("to ")) {
const d = dir.slice(3).trim();
if (d === "right") {
x0 = 0;
y0 = 0;
x1 = width;
y1 = 0;
} else if (d === "left") {
x0 = width;
y0 = 0;
x1 = 0;
y1 = 0;
} else if (d === "bottom") {
x0 = 0;
y0 = 0;
x1 = 0;
y1 = height;
} else if (d === "top") {
x0 = 0;
y0 = height;
x1 = 0;
y1 = 0;
}
} else {
parts.unshift(dir);
}
const grad = ctx.createLinearGradient(x0, y0, x1, y1);
const colorStops = parts;
const n = Math.max(1, colorStops.length - 1);
for (let i = 0; i < colorStops.length; i += 1) {
const color = extractColorFromStop(colorStops[i] as string);
grad.addColorStop(Math.min(1, Math.max(0, i / n)), color);
}
ctx.fillStyle = grad;
ctx.fillRect(0, 0, width, height);
}
function drawRadialGradient(
ctx: CanvasRenderingContext2D,
width: number,
height: number,
layer: string
): void {
const inside = layer.slice(layer.indexOf("(") + 1, layer.lastIndexOf(")"));
const parts = splitBackgroundLayers(inside);
const first = (parts.shift() || "").trim();
let cx = width / 2;
let cy = height / 2;
if (first.startsWith("circle at")) {
const pos = first.replace("circle at", "").trim();
const [px, py] = pos.split(" ");
const parsePos = (p?: string, full?: number): number => {
if (!p) return (full || 0) / 2;
if (p.endsWith("%")) return (parseFloat(p) / 100) * (full || 0);
if (p === "left") return 0;
if (p === "right") return full || 0;
if (p === "top") return 0;
if (p === "bottom") return full || 0;
if (p === "center") return (full || 0) / 2;
return (full || 0) / 2;
};
if (px && py) {
cx = parsePos(px, width);
cy = parsePos(py, height);
} else if (px) {
cx = parsePos(px, width);
cy = parsePos(px, height);
}
} else {
parts.unshift(first);
}
const r = Math.hypot(width, height);
const grad = ctx.createRadialGradient(cx, cy, 0, cx, cy, r);
const colorStops = parts;
const n = Math.max(1, colorStops.length - 1);
for (let i = 0; i < colorStops.length; i += 1) {
const color = extractColorFromStop(colorStops[i] as string);
grad.addColorStop(Math.min(1, Math.max(0, i / n)), color);
}
ctx.fillStyle = grad;
ctx.fillRect(0, 0, width, height);
}
export function drawCssBackground(
ctx: CanvasRenderingContext2D,
width: number,
height: number,
css: string
): void {
const layers = splitBackgroundLayers(css).filter(Boolean);
for (let i = layers.length - 1; i >= 0; i -= 1) {
const layer = layers[i] as string;
if (layer.startsWith("linear-gradient(")) {
drawLinearGradient(ctx, width, height, layer);
} else if (layer.startsWith("radial-gradient(")) {
drawRadialGradient(ctx, width, height, layer);
} else if (
layer.startsWith("#") ||
layer.startsWith("rgb") ||
layer.startsWith("hsl")
) {
ctx.fillStyle = layer;
ctx.fillRect(0, 0, width, height);
}
}
}
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